Enhancing chemical durability of PFSA for heavy duty vehicle applications using cerium immobilizers and organic additives
Date
2023
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
NafionTM belongs to a class of ion-conducting polymers known as perfluoro sulfonic acid (PFSA). NafionTM is the state-of-the-art polymer electrolyte membrane (PEM) used in hydrogen-oxygen fuel cells. However, NafionTM undergoes chemical degradation over time due to harmful peroxide and hydroxyl radicals formed during the fuel cell reaction. These radicals attack Nafion’s side chains and cause polymer unzipping thus degrading the membrane’s mechanical integrity over time. Protecting the membrane from such degradation is particularly important in fuel cells for heavy-duty vehicle applications where life expectancy can exceed 1 million miles. Cerium, when incorporated within the membrane, has proven to be an effective radical scavenger owing to its rapid transition between +3 and +4 oxidation states, and its swift regeneration in the fuel cell environment. However, over time, cerium migrates out of the membrane due to the electrochemical potential, high water flux, and ionic diffusion. To mitigate cerium migration, we investigated a wide class of organic cerium immobilizers, and measured their effectiveness in terms of cerium retention, radical scavenging activity, and fuel cell performance. We found that ligands with an optimal structure could help retain over 80% of cerium compared to 2% retention for the baseline membranes without immobilizers, without degrading fuel cell performance, or reducing cerium radical scavenging activity. ☐ Since crown ether showed promise as a cerium immobilizer, we synthesized an organometallic complex of cerium with 15-Crown-5 and investigated its effectiveness to immobilize cerium. Over 1000% increase in cerium retention and 80% increase in chemical durability was observed owing to the stabilization effect of crown ethers on cerium. We found that migration under potential gradient could be eliminated while the complex also contributed to enhancement in cerium radical scavenging activity. ☐ Phosphonic acids, which appeared to be the best class of materials for cerium immobilization based on an initial screening study were selected for a full investigation. It was found that phosphonic acids also showed radical scavenging activity comparable to cerium but without the migration issue. The ex-situ Fenton test showed that the fluoride emission rate (FER) of fluoroalkyl phosphonic acid-incorporated NafionTM membranes ranged from 0.22-0.37 µg F cm−2 hr−1 , lower than that of the cerium-incorporated NafionTM membrane’s FER of 0.39 µg F cm−2 hr−1 . The in-situ open circuit voltage hold test confirmed that the phosphonic acid-incorporated NafionTM membrane has a 58% lower FER compared to the cerium-incorporated membrane. Density functional theory calculations indicated that the activation energy of the hydroxyl radical scavenging reaction of an alkyl phosphonic acid is only 0.68 eV, suggesting an effective radical scavenging pathway. ☐ Since water-soluble cations have a natural tendency to migrate through the hydrated membrane, alternate water-insoluble organic radical scavengers that could be as effective as cerium were then investigated. Organic radical scavengers could be better stabilized by their interactions with Nafion’s hydrophobic backbone. Potent organic scavengers could also be chemically bonded to the NafionTM side chain to prevent their migration. Tens of potential organic additives were screened and the most promising candidate was found to be Ellagic acid. Ellagic acid (EA) is an effective organic radical scavenger for PFSAs due to its chemical stability, radical scavenging ability comparable to state-of-the-art antioxidants, and high reversibility in fuel cell operating conditions. The incorporation of EA increased the chemical lifetime of NafionTM by 160% without the use of cerium. When EA was incorporated in NafionTM along with cerium, the OCV hold lifetime surpassed 900 hr. after which the test was discontinued. In comparison, a membrane with just cerium showed a lifetime of 500 hr. This is the first report of such high radical scavenging with an organic radical scavenger. ☐ This dissertation has investigated strategies to eliminate issues with cerium as a radical scavenger ranging from immobilizing it to completely replacing it with alternative materials. While the results from this work are very promising, future work is needed in this area to further extend the life of fuel cell membranes in heavy-duty vehicle applications.
Description
Keywords
Perfluoro sulfonic acid, Organic additives, Cerium immobilizers, Heavy duty, Polymer electrolyte membrane